Oak for me. But some words on each of them:
Oak - the original professor, and mentioned a lot of times to be a leader in his field. Sure, there's some nostalgia in me liking him (even if GSC were my first games!), but he's the one we've seen the most of, and is reputable enough to go around the world and work with other professors and even run a radio show.
Elm - 'durr I have a child and am the authority on Pokemon breeding what is this egg where did it come from!?'. Sorry, Elm.
Birch - an outdoorsy guy seems interesting, but... being attacked by a wild Zigzagoon and being a professor and not having Pokémon on hand... Sort of breaks that golden rule you first heard from Oak, no?
Rowan - grumpy old fart, but he seemed to know his stuff and was also entertaining in parts in Platinum.
Juniper - also went about on her studies, but didn't go abut without Pokémon like Rowan. I liked how she tells it like it is to N as well, who I didn't really like, so plus points for me.
Sycamore - indifferent to professor mcfabhair. He seems a bit too easy-going about the whole Lysandre plot though. 'Oh that silly guy, making a doomsday machine like that' uh.